Debonding at the fibre-matrix interface under remote transverse tension. One debond or two symmetric debonds?

The controversy about the symmetry of the debond onset at the fibre-matrix interface in single-fibre specimens under transverse tension is studied here applying the coupled stress and energy criterion of the finite fracture mechanics. This criterion enables to compare the two different post-failure configurations found in the literature studying this problem: an asymmetric configuration with a single debond and a symmetric one with two debonds. The coupled criterion applied here predicts that an asymmetric post-failure configuration is originated by a lower critical remote tension than the symmetric one, the difference being above 10% in some cases. Thus, the asymmetric debond onset is the preferred solution to the initially symmetric problem, which agrees with the experimental evidences found in the literature. This result is a consequence of the shielding effect between the two debonds in the symmetric solution. Numerical results are obtained for two common composites, glass/epoxy and carbon/epoxy, and three virtual bimaterials corresponding to the extreme values of the Dundurs elastic parameters. The stress-strain curves predicted for the two post-failure configurations are also compared. Eventually, simplified models using springs and breakable elements, which capture the essence of the problem studied and explain the energetic origin of the loss of symmetry, are introduced.